Why does unordered_set::begin() change if no rehashing occurs?

Question

Based on reading https://en.cppreference.com/w/cpp/container/unordered_set/emplace , it states: Rehashing occurs only if the new number of elements is greater than max_load_factor()*bucket_count().

In this code:

int main() 
{ 
    unordered_set<int> myset;
    myset.emplace(4);
    cout << myset.bucket_count() << endl;
    cout << myset.max_load_factor() << endl;

    auto it = myset.begin();
    myset.emplace(3);
    cout << (it == myset.begin()) << endl;
}

There is no rehashing when I do emplace the second time (bucket count is 2 and max load factor is 1 right before emplacing the second time, which is not more than the new number of elements, 2), but my begin() iterator changes. Why is this iterator invalidated/changed even though there is no rehashing?

Answer 1

As john already mentioned, your iterator is still valid but it points to the begin() of your unordered_set container at the moment where there was only one element '4'.

So, one thing you should keep in mind using C++ iterators - don't cache them at all. Unless you don't forget to update the iterator every time you do change the container invariants.